Well, it is in the ports tree as x11/kde4 ... though it is not in x11/Makefile, and comes with the following README:
This is completely experimental! This is here so that people can
play with it.
Things *will* break! This is totally expected. Dependencies are still
There are issues in KDE proper (it's released, but it's still early
days yet for KDE4), and some issues related to OpenBSD.
Also, this KDE4 will leave you with quite a few processes lying around
when you've played with it.
It should be obvious, but the list of things to be done is too long to
maintain; just find something that doesn't work for you, fix it, and
post a "cvs diff -u" to ports@.
I've never tried building it, myself. The last direct discussion in ports@ was 13 month ago.